In gas analysis, it is frequently desirable to monitor a time-dependent concentration of an analyte in a sample gas. In such situations, the “memory effect” is a well-known problem in the art, in which the measured analyte concentration can undesirably depend on the past history of the measurement system. For example, if the analyte concentration starts out high and then abruptly decreases to a much lower level, the gas analysis sensor may provide erroneously high analyte readings for some time after the analyte concentration decreases. The memory effect is especially severe for sticky analytes, such as water vapor or ammonia.
Various physical processes can contribute to the memory effect. For example, the analyte can be adsorbed onto surfaces of the sample chamber and/or gas delivery system. The analyte can also be dissolved (i.e., absorbed) into parts of the sample chamber and/or delivery system.
Several methods of mitigating or avoiding this memory effect have been considered in the art. For example, in U.S. Pat. No. 5,661,038, isotopic analysis of Hydrogen is improved by providing a water trap to remove memory effects due to water vapor. In U.S. Pat. No. 6,310,340, memory effects from a leakage valve part of the gas delivery system are mitigated by using this part of the system as a barrier gas line and/or as a pumping line. In U.S. Pat. No. 5,841,022, two parallel input systems are employed in alternating succession, such that the gas input system that is idle at any point in time can be purged to remove absorbed/adsorbed gas.
In U.S. Pat. No. 5,454,860, mitigation of a somewhat different memory effect is considered in a system in which the sample is provided as a liquid and made gaseous for analysis. The mitigation approach taken in this reference is to actively pump out fluid from the system, thereby reducing memory effects due to lingering fluid.
However, a need remains for improved mitigation of the memory effect associated with gas absorption/adsorption in the gas handling system. This need is especially acute in cases where the analyte of interest is a sticky gas that tends to have significant adsorption/absorption, such as ammonia or water vapor.